1. Field of the Invention
This invention relates to a strongly correlated nonvolatile memory element. More specifically, this invention relates to a strongly correlated nonvolatile memory element which exhibits nonvolatile memory functions by electrical means.
2. Related Art
In recent years there have been concerns that the scaling rule which has been a guiding principle for improvement of semiconductor device performance will finally reach a limit. These concerns have been accompanied by work to develop materials which make possible novel operation principles to overcome the crisis of limits to transistor performance. For example, in the field of spintronics which incorporate spin freedom, development is underway with the goal of high-density nonvolatile memory capable of fast operation comparable to DRAM (Dynamic Random Access Memory).
On the other hand, there has also been progress made in research on materials having strongly correlated electron systems, to which the band theory which has supported the foundations of semiconductor device design cannot be applied. As a result of this progress, materials have been discovered which exhibit huge and rapid changes in physical properties arising from phase transitions of electron systems. In strongly correlated electron system materials, the degrees of freedom of electron orbitals as well as spin contribute to the phase state of the electronic system, and consequently various electronic phases, indicating diverse orders formed by spin, electric charge, and orbitals, are exhibited. Perovskite manganite is a representative example of a strongly correlated electronic system material; it is known that due to first-order phase transitions, this electronic system exhibits a charge-ordered phase in which 3d electrons of manganese (Mn) are ordered, and an orbital-ordered phase in which electron orbitals are ordered.
In the charge-ordered phase and the orbital-ordered phase, carriers are localized, so that electrical resistance is increased, and the electronic phases are insulator phases. Further, the magnetic properties of these electronic phases are antiferromagnetic phases due to double exchange interactions and superexchange interactions. The electronic states of charge-ordered phases and orbital-ordered phases often should be regarded as semiconducting. This is because in charge-ordered phases and orbital-ordered phases, carriers are localized, but electric resistivity is lower than that of so-called band insulators. However, here the convention is adopted that the electronic phases of charge-ordered phases and orbital-ordered phases are insulator phases. Conversely, when electrical resistance is low and metal-like behavior is exhibited, spins are aligned, and so the electronic phase exhibits ferromagnetism. There are various definitions of metallic phases, but in the present application, a metallic phase is regarded as one for which “the sign of the differential temperature coefficient of resistivity is positive”. Corresponding to this, the above-described insulator phase may be re-defined as one for which “the sign of the differential temperature coefficient of resistivity is negative”.
Further, two-phase coexistence states, in which a metallic phase and an insulator phase coexist in a material, are also known. The electrical resistance values of materials which exist in a two-phase coexistence state are determined by percolation in the metallic phase in the material. For example, if between two electrodes provided in the material to measure the electrical resistance, metallic phase regions are connected to form even a single path, the resistance is reduced, whereas if such a path does not exist, the resistance is high. Whether a material in such a two-phase coexistence state behaves as a conductor or as an insulator, that is, the apparent electrical resistance value of the material, is determined by the geometric structure of paths of the metallic phase (typically, lengths, widths and number) brought about by such percolation. It is known that electrical resistance values can take on two or more values, according to the geometric structures of paths of metallic phase regions in a two-phase coexistence state.
In addition to the above-described charge-ordered phase and orbital-ordered phase, various switching phenomena have been reported in single-crystal bulk samples of materials which can enter an electronic phase in which both charge-ordering and orbital-ordering obtain (a charge- and orbital-ordered phase). See Japanese Patent Application Laid-open No. H8-133894 (also referred to herein as “Patent Document 1”), Japanese Patent Application Laid-open No. H10-255481 (also referred to herein as “Patent Document 2”) and Japanese Patent Application Laid-open No. H10-261291 (also referred to herein as “Patent Document 3”). Such switching phenomena are exhibited according to application of such stimuli as temperature changes surrounding a transition point, application of a magnetic field or electric field, or optical irradiation. The switching phenomena are typically observed as giant changes in electrical resistance, and phase transitions between an antiferromagnetic phase and a ferromagnetic phase. For example, changes in resistance by several orders of magnitude due to application of a magnetic field are well known as the colossal magnetoresistance effect.
For some time now research has been conducted on electric field effect elements using such strongly correlated electron system materials as thin films in channel layers. For example, it is reported that when using an La0.7Ca0.3MnO3 thin film as a channel layer, and fabricating a ferroelectric PbZr0.2Ti0.8O3 (PZT) thin film thereupon as a gate insulator, nonvolatile resistance changes in the channel layer are induced by remanent polarization of the ferroelectric PbZr0.2Ti0.8O3 thin film. See S. Mathews et al., “Ferroelectric Field Effect Transistor Based on Epitaxial Perovskite Heterostructures”, Science, vol. 276, 238 (1997) (also referred to herein as “Non-Patent Document 1”). Non-Patent Document 1 reports that the resistance of the channel layer is lowered by application of a positive voltage, and is increased by application of a negative voltage. Using the fact that first-order transitions are possible in a single-crystal thin film on a (110) plane oriented substrate (see Japanese Patent Application Laid-open No. 2005-213078, also referred to herein as “Patent Document 4”), a pn junction has also been reported which uses a thin film of Nd0.5Sr0.5MnO3, which is a strongly correlated oxide thin film exhibiting metal-insulator transitions, as the p layer, and Nb-doped SrTiO3 (110) substrate as the n layer. See J. Matsuno et al., “Magnetic field tuning of interface electronic properties in manganite-titanate junctions”, Applied Physics Letters, vol. 92, 122104 (2008) (also referred to herein as “Non-Patent Document 2”). And recently, research has been reported on a three-terminal element using an NdNiO3 thin film exhibiting metal-insulator transitions as a channel layer. See S. Asanuma et al., “Tuning of the metal-insulator transition in electrolyte-gated NdNiO3”, Applied Physics Letters, vol. 97, 142110 (2010) (also referred to herein as “Non-Patent Document 3”).
However, according to Non-Patent Document 1, the amount of change in resistance occurring over an applied voltage range of ±10 V, expressed as a ratio, is no greater than approximately threefold. Non-Patent Document 1 uses a ferroelectric as the gate insulator. Hence the lattice mismatch at the interface between the manganese oxide used in the channel layer and the ferroelectric material which is the gate insulator thereabove becomes too great, and the occurrence of defects in the interface is unavoidable. If such defects are introduced, leakage currents through the gate insulator occur and the ferroelectric properties themselves are degraded, so that there are concerns that problems will occur if the device is used over a long period of time. Above all, the nonvolatility occurring in Non-Patent Document 1 utilizes the ferroelectric properties of the gate insulator. That is, the ferroelectric gate insulator is essential to realize nonvolatility. Further, in the pn junction described in Non-Patent Document 2 also, the giant changes in current density or capacitance expected from resistance changes of five or more orders of magnitude due to metal-insulator transitions in Nd0.5Sr0.5MnO3 thin film and similar have not been observed, and nonvolatility is not reported. And, in the report of Non-Patent Document 3 also, although the metal-insulator transition temperature was lowered approximately 40 K by application of a −2.5 V gate voltage to a sample of channel layer thickness 5 nm, complete transition to a metallic phase due to the gate voltage did not occur, and nonvolatility is not reported. Thus in disclosures and reports to date, nonvolatile memory functions utilizing giant resistance changes have not been attained in electric field elements and other circuit components or elements using strongly correlated oxides in channel layers.
In addition, there are no known methods for using a two-phase coexistence state of a metallic phase and an insulator phase and the accompanying percolation in the operation of an element using a strongly correlated oxide in a channel layer, or in the operation of nonvolatile memory.